If I were you I would read the above link then tell your boss you quite rightly have not decided exactly when you are going on mat leave but will tell him in plenty of time and same with coming back. It makes me mad when I hear things like this, your boss has no right to demand answers to questions like this and yes he does have to leave your job open for you
I can't see what the problem is with your boss asking you when you intend to go off on maternity leave? Perhaps he's only trying to plan ahead to get cover? If you don't know yet then just tell him that.
Yes, they do have to keep your job open for you in most cases unless, for instance, you are made redundant, which you can be whilst on maternity leave. If there are redundancies you have to be given priority for any suitable alternative vacancy however.
You leave when you're ready to anddon't let him pressure you - you have the law on your side. If you want to change the date you give them, you can if you give 28 days notice.
Statutory maternity pay is paid by the government and they can't touch that. Check your employment contract to see if you entitled to any extra payments from your employer. If it's in your contract them they cannot refuse to pay up.
If you don't want to go back to work, you have to give your normal notice period, so there's no need to commit yourself now.